1. Field of the Invention
This invention generally relates to non-synchronous digital communications and, more particularly, to a system and method for determining a non-predetermined input data rate of a serial communications stream.
2. Description of the Related Art
A synchronous communications network digital payload data is carried on a particular clock frequency within a synchronous message format. This payload data may include both asynchronous digital data and synchronous digital data originating at a different data rate in a foreign digital network. The Synchronous Optical Network (SONET) and its European counterpart the Synchronous Digital Hierarchy (SDH) provide a standard format of transporting digital signals having various data rates, such as a DS-0, DS-1, DS-1C, DS-2, or a DS-3 signal and their European counterparts within a Synchronous Payload Envelope (SPE), or a container that is a part of a SONET/SDH STS-N/STM-N message frame. In addition to the digital data that is mapped and framed within the SPE or container, the STS-N/STM-N message frame also includes overhead data that provides for coordination between various network elements.
As is known, a phase-locked loop (PLL) is used to recover the clock and data signal, and is able to smooth out some phase jumps caused by pointer adjustments or asynchronous stuff bits. A typical optical network transceiver or PHY unit is expect to be able to operate at a number of different data rates, depending upon the network signal protocol being used. The optical transceiver may be hardcoded or hardwired to operate at a particular predetermined frequency, but this process requires user intervention, and limits to unit to operating at only one frequency. Alternately, the transceiver can be designed to interact with the backplane, which “knows” the optical data rate, and change frequencies in response to an optical transceiver-to-electrical backplane protocol. However, the use of such a protocol adds to the cost and complexity of the transceiver. As another alternative, the optical transceiver can be designed to acquire a non-predetermined optical data rate independent of communications with the backplane. Again however, such an alternative needlessly adds to the cost and complexity of the transceiver, since the backplane knows the optical data.
It would be advantageous if an optical transceiver could acquire a non-predetermined input data rate, independent of user intervention or backplane communications, by checking the input signal against a finite list of candidate frequencies.